There are various types of wastewater disposal systems in use such as municipal water treatment systems. In some situations, typically in rural areas where houses are spaced so far apart that a municipal or shared sewer system would be too expensive to install, people install their own treatment systems. These systems usually employ septic tanks. A septic tank system is simply a tank that is usually buried. The tank might hold 1,000 gallons (4,000 liters) of water. Wastewater flows into the tank at one end and exits the tank at the other end, typically into a leach field. Septic tanks often include an opening on the top surface which is meant to provide access to the tank in order to facilitate cleaning and maintenance of the tank.
Septic tanks are typically constructed from concrete, steel, fiberglass or plastic. Plastic septic tanks are typically constructed using rotational molding techniques. In contrast, the present invention uses blow molding, which is a plastic manufacturing process that manufactures hollow parts that can be irregular in shape. Examples include air ducts for cars, tool cases, toys, furniture and bottles/containers.
A variety of Federal, state and local regulations impose requirements for septic tank construction. Among other things, these requirements demand a watertight and structurally sound vessel. A monolithically manufactured poly tank using previous construction methods easily meets the watertight requirement.
The problems with existing poly tanks usually manifest in the structurally sound category. A typical problem associated with existing poly tanks is collapse of the tank or deformation of vessel walls resulting from the pressure earth of the fill material surrounding the tank. Because most poly tank are manufactured using a corrugated wall profile, which typically runs circumferentially around the tank, the existing poly tanks act like an accordion when subjected to the earth's pressures, a term know in the trade as “creep”.
The process of manufacturing rotationally molded tanks uses centrifugal force to spread poly resin against the mold walls which results in great variation and relatively little control in wall thickness distribution. Additionally, rotational molding is limited in its ability to manufacture parts that have significant sections with solid walls or structural stiffeners because the process cannot force poly resin into deep cavities within the mold.
The blow-molding process is a pressure process whereby poly resin is forced under pressure to seek the limits of a mold without respect to gravitational or centrifugal forces. The viscosity of the poly resin is such that this pressure can force the material into comparatively deep cavities integral to the wall structure. Hence, stiffening ribs and flanges can be integrally molded into the vessel walls to provide an increase in structural integrity.